The invention relates to electrostatic filtration media for gas filtration, and more particularly to various combinations of synthetic fiber media of different cross-sectional shapes with and without charge control agents, and methods of making fibers.
It is known in the filtration art that various kinds of fibers can be formed into a web or other nonwoven structure having tortuous paths between the fibers through which a gas stream, such as air, is passed to remove particulate matter from the gas stream. The particulate matter in the gas flowing through the paths in the web is retained on the upstream side of the web, or within the tortuous paths of the web due to the size of the particles relative to the paths' diameters.
Synthetic fibers come in various cross-sectional shapes including, but not limited to, round, flat, trilobal, kidney bean, dog-bone, bowtie, ribbon, 4DG (multiple grooves along a fiber's length), hollow, sheath core, side by side, pie wedge, eccentric sheath core, islands and three island. It is known to charge various blends of fibers electrostatically to further retain particulate matter through electrostatic attraction between the fibers and the particles. Such blends and other filtration improvements are shown in U.S. Pat. No. 6,328,788 to Auger, U.S. Pat. No. 4,798,850 to Brown, U.S. Pat. No. 5,470,485 to Morweiser, et al., and U.S. Pat. No. 5,792,242 to Haskett, all of which are incorporated herein by reference.
Electrostatic (“electret”) filter media have increased efficiency without necessarily increasing the amount of force it takes to push the air though the filtration media. The “pressure drop” of media is the decrease in pressure from the upstream side of the media to the downstream side. The more difficult it is to force air through the media, the greater the pressure drop, and the greater use of energy to force air through the media. Therefore, it is generally advantageous to decrease pressure drop.
Electrostatically charged media is used in many different applications and in many different filters. The electrostatic charge is applied either by oxidatively treating the fibers, such as by passing the fibers through a corona, or by triboelectric charging, which is a type of contact electrification in which certain materials become electrically charged after they come into contact with a dissimilar material (such as through rubbing) and are then separated. The polarity and strength of the produced charges differ according to the materials, surface roughness, temperature, strain, and other properties. Such electrostatic charges tend to dissipate over time, and this leads to reduced efficiency in removing particles from the air stream. In particular, the efficiency of removing small particles (below 10 microns) is reduced as charges dissipate.
Currently, passive electrostatic filter media works mainly on the principle of friction from air passing over fibers and inherent static electricity in the polymer itself. Static electrification is not caused merely by air flow impacting a solid surface. In many cases a polypropylene honey-combed netting, typically with woven monofilament approximately 0.01 inches in diameter, is used in different layers or in conjunction with a urethane foam or a high loft polyester which also has inherent static charges. In the more efficiently static charged fiber filters, each fiber has both positive and negative charges. Such fibers are 1 to 30 denier in size, or approximately 0.00049 to 0.00268 inches in diameter. Generally these types of filters are more expensive than a disposable type, lasting about three months in a residential heating, ventilation and air conditioning (HVAC) unit. The efficiencies of such electret filters are known to be very good, with approximately 34% to 40% dust spot efficiency.
Charge control agents have been used to control static charges in imaging applications, such as photostatic and Xerographic (electrophotographic) processes. Positive and negative charge-control agents, charge directors, charge additives, charge adjuvants, zwitterionic materials, polar additives, dielectric substance and electroconductivity-providing agents, (ECPA's), all are referred to as charge control agents (CCA's) and are known for use in the following applications with some CCA's on some materials: powder toner, powder coating or powder electrophotographic toner, liquid toner, liquid developer, chargeable toner, electrophotographic recording, powder paint, developing toner, electrostatic copying, electret fiber material, electrophoretic image display, impulse ink jet printing, sprayable powder coating and electrostatic image developer.
The prior art filtration materials provide sufficient filtration for many environments. However, where the electrostatic charge must be retained for long periods or where the penetration through the filtration material must be below a particular percentage, the prior art does not suffice, or is prohibitively expensive to manufacture. Therefore, the need exists for a filtration material that provides the needed performance at a feasible cost to manufacture.